disintegration at LHCb

Transcription

1 Study of ψ(2s) J/ψ η disintegration at LHCb TP4 Project January 7, 2011, first version Laureline Josset Date: January 7, 2011 Master EPFL Section Physique TP4 Report Professor T. Tatsuya Assistant F. Blanc

2 Contents 1 Introduction Aim Motivations LHC & the LHCb experiment LHC LHCb experiment B mesons ψ(2s) J/ψ η ψ(2s) particle properties J/ψ particle properties η particle properties Choice of decay mode Preliminary Analysis on signal Monte Carlo ψ(2s) with η π + π π MC Data Construction of the dataset and first selections MC Results for ψ(2s) with η π + π π Pseudo η π + π reconstruction Construction of the dataset and selection MC Results for ψ(2s) with η decaying in 2 pions First Conclusions Monte Carlo simulation studies η π + π Data Selection cuts Result and interpretation η π + π π Data Selection cuts Result and interpretation η γ γ Data Selection cuts Result and interpretation

3 CONTENTS 3 6 Real Data Data η π + π Selection cuts Result Investigation on ψ(2s) J/ψ π + π Approximation on the number of ψ(2s) J/ψ η events η π + π π Selection cuts Result and interpretation η γ γ Selection cuts Result and interpretation Conclusion 34 8 Annexe 35

4 Chapter 1 Introduction The general frame of this study is the LHCb detector mainly devoted to the beauty quark and the violation of the CP symmetry. This project focuses on the preliminary analysis necessary for the study of CP in channels containing ψ(2s) J/ψ η decays. 1.1 Aim The aim is to get familiar with the general functioning of the LHCb, the simulation process, and mainly on the analysis tools. The other aspect of the study is the understanding of the particle physics behind the disintegration. In practical terms, the goal is to find ψ(2s) particle decaying in J/ψ and η in the latest available LHCb data from 2010 (Stripping10). This is done in three steps : first, a preliminary analysis is conducted on a limited and mixed set of simulated data corresponding to a particular B 0 s decay channel. Secondly, the possible cuts and parameters are studied in depth in the case of simulated data for each disintegration mode. Finally, the chosen selection cuts are applied on real data. 1.2 Motivations As previously mentioned, the goal is to observe ψ(2s) J/ψ η. In a general context, LHCb experiment studies particle physics involving the b-quark. This project restrains its field of study to the ψ(2s) particle. Further work is to combine this particle with φ to reconstruct B 0 s and conduct deeper analysis on its properties. 4

5 Chapter 2 LHC & the LHCb experiment This chapter briefly introduces the LHC and the LHCb experiment. It also shortly presents the bigger scheme behind the study of particles part of B mesons disintegration channels. 2.1 LHC The Large Hadron Collider (LHC) is the largest experimental device ever built to validate physical theories. It is primarily designed to accelerate bunches of protons to an energy of 7 [T ev ] (14 [T ev ] center of mass). The LHC has a design luminosity of [cm 2 s 1 ] and a bunch-crossing frequency of 40 [MHz]. Four main detectors are located at the crossing-points of the main ring: ATLAS, CMS, LHCb and ALICE. Apart from the two general-purpose detectors, CMS and ATLAS, each experiment is designed with different physics aims in mind. Operating Conditions The LHC has produced proton-proton collisions for sustained periods of time at its optimum energy since the beginning of The nominal LHC beam conditions are given in Table 2.1. All simulated data used in this project are simulated to reproduce LHCb operating conditions. The 2010 data used for this analysis were acquired at 3.5 TeV per beam, but at reduced luminosity. Operating conditions Energy per beam 3.5[T ev ] Number of protons per bunch Luminosity 2 x β 0.5 m Table 2.1: Nominal parameters of the LHC beam. 2.2 LHCb experiment The Large Hadron Collider beauty (LHCb) experiment at CERN is a forward one-arm spectrometer. Its design reflects the predicted distribution of b-quarks produced at the LHC. The detector covers a polar angle of 25 to 300 [mrad] in the horizontal direction and 25 to 250 [mrad] in the vertical direction. LHCb aim is to study CP violation and other rare phenomena in the decay of hadrons containing b-quarks at the LHC. Other physics goals are to check the consistency of the Standard Model through precise measurements and to search for new physics. 5

6 6 CHAPTER 2. LHC & THE LHCB EXPERIMENT 2.3 B mesons B mesons are at the center of LHCb research because they are of particular interest to study CP violation. B is a bottom meson, that is to say a particle with one quark and one anti-quark, where one of them has a beauty flavor. CP violation can be direct or indirect: - B 0 (s) B0 (s) oscillations Neutral B d or B s can transform into their antiparticles and vice versa, but such transformation does not occur with exactly the same probability in both directions. This is CP violation in the mixing. It is incorporated in the Standard Model by including a complex phase in the Cabibbo Kobayashi Maskawa (CKM) matrix describing quark mixing. By studying the life-time of the particle before decaying, one can deduce if the particle has oscillated. Feynman diagram of the oscillation mentions CKM matrix factors. If difference is shown between the two cases, B 0 B 0 or B 0 B 0, CP violation can be evaluated. It can also occur in the interference between mixing and decay. - Asymmetry between B + and B CP violation can be observed by comparing the B + and B branching fractions to a specific decay channel. This is called direct CP violation. Its interpretation is complicated by QCD effects. This project focuses on ψ(2s) particle with the bigger objective to combine it with other mesons to reconstruct B 0 s particles and get additional results on CP violation.

7 Chapter 3 ψ(2s) J/ψ η This chapter details the properties of the different particles and decay channels at play behind the reconstruction of ψ(2s). 3.1 ψ(2s) particle properties ψ(2s) is a heavy unflavored meson. It is an excited state of J/ψ, a cc charmonium meson. Main decay channels for ψ(2s) Properties of ψ(2s) Quark composition cc Full width Γ = 304±9 [kev] Quantum numbers I G (J P C) = 0 (1 ) Mass ± 0.04 [MeV] ψ(2s) decays mainly in the following final states: ψ(2s) decay modes ψ(2s) J/ψ π + π (33.6 ± 0.4)% J/ψ π 0 π 0 (17.73 ± 0.34)% J/ψ η (3.28 ± 0.07)% Table 3.1: ψ(2s) decay channels In the present study, not all modes of disintegration for ψ(2s) are considered. The focus is only put on ψ(2s) J/ψ η This disintegration channel is slightly more exotic than others. Especially, it is ten time less important than J/ψ π + π. This will appear later in this report. 3.2 J/ψ particle properties Properties of J/ψ Quark composition cc Full width Γ = 92.9±2.8 [kev] Quantum numbers I G (J P C) = 0 (1 ) Mass ± [MeV] 7

8 8 CHAPTER 3. ψ(2s) J/ψ η Disintegration of J/ψ J/ψ decay modes J/ψ e + e (5.94 ± 0.06)% J/ψ µ + µ (5.93 ± 0.06)% For the reconstruction of J/ψ, the muon channel only is considered. They are detected in the muons stations placed at the far end of the detector. This leads to a good reconstruction of the mother particle without too much combinatory effects. 3.3 η particle properties Disintegration of η Properties of η Quark composition c1(uu + dd) + c2(ss) Full width Γ = 1.30±0.07 [kev] Quantum numbers I G (J P C) = 0 + (0 + ) Mass ±0.024 [MeV] Main decay modes for η η γγ (39.31 ± 0.2)% π 0 π 0 π 0 (32.57 ± 0.23)% π + π π 0 (22.74 ± 0.28)% π + π γ (4.6 ± 0.16)% In the present case, not all decay channels in the above table for η are considered. The two last modes are the most promising, thanks to the two charged pions and despite the presence of a neutral particle in each of them. Between the two modes, the first one is kept as its branching ratio is 5 times more important. The 3π 0 mode is rejected. Indeed, neutral particles are detected in the calorimeters, where no information on the direction of the tracks is available. In addition, their reconstruction is subject to many combinatory effects, creating a large background. This channel is subject to too many errors and thus put aside. The abundance of γ generated in the pp collision is the source of many combinations. This mode is kept because of the large branching ratio. 3.4 Choice of decay mode The precedent considerations on the mode of disintegration lead to the following choice: ψ(2s) J/ψ η µ + µ π + π π 0 γγ

9 Chapter 4 Preliminary Analysis on signal Monte Carlo ψ(2s) with η π + π π 0 The first step of the study is to perform a preliminary analysis to define whether it is possible to observe the particle ψ(2s) and under which conditions. The simulated signal corresponds to the geometrics and characteristics of the detector. The purpose of this preliminary analysis is to give leads for more detailed studies on the disintegration. 4.1 MC Data Preliminar analysis are conducted on Monte Carlo generated data for the disintegration: B 0 s φ ψ(2s), where ψ(2s) decays into J/ψ η, where the η mesons can decay into either of the considered channels. The decay of B 0 s via φ ψ(2s) represents (6.8 ± 0.27) 10 4 of B 0 s disintegration. MC2010 at 7 TeV in center of mass with the full detector is used in the present case, allowing comparison with real data. The dataset for signal events analysis contains events and is constructed through C++ and python codes at DaVinci level. 4.2 Construction of the dataset and first selections From DaVinci physical analysis, the particles, tracks and vertex of interests are taken to construct the dataset. The table in figure 4.1 details the criteria applied for the selection. These criteria are applied on the transverse momentum, on the quality of the vertices and on invariant masses. Input particle lists for the reconstruction are mass constrained J/ψ from µ + µ, standard loose pions for π + and π and standard loose resolved π 0. 9

10 10CHAPTER 4. PRELIMINARY ANALYSIS ON SIGNAL MONTE CARLO ψ(2s) WITH η π + π π 0 Figure MC Results for ψ(2s) with η π + π π 0 First results for the 3 pions channel are shown in figure 4.2. No ψ(2s) signal (M = 3686 MeV) can be seen in the mass distribution of the particle. Figure 4.2: ψ(2s) mass distribution J/ψ mass distribution shows a clean peak of 30 MeV wide - figure 4.3. There is nothing to improve for that particle.

11 4.3. MC RESULTS FOR ψ(2s) WITH η π + π π 0 11 Figure 4.3: J/ψ mass distribution On the other hand, η (M = 548 MeV) is dominated by background - figure 4.4. Indeed, its mass distribution curve is just as unclear as ψ(2s). Figure 4.4: η mass distribution Looking at the pions, one can notice that the problem comes from the π 0. It is randomly distributed within the imposed mass window - figure 4.5. This was expected and is most certainly at the origin of all problems reconstructing η π + π π 0. One can also observe that for events, there are POTIRON entries. A selection on the best candidate for each event could be efficiently removing random combinations. Monte Carlo simulation keeps information on the particles real identity. Recovering the true signal data from the Monte Carlo Truth variables, the figures 8.1 and 8.2 shown in annexe are created. One can observe that the particles momentum and transverse momentum in ψ(2s) rest frame have low values. Indeed, the sum of the daughter particles masses is very close to ψ(2s). M ψ(2s) = 3686 MeV M J/ψ + M η 3643 MeV Cuts on momentum should then not be reinforced too much.

12 12CHAPTER 4. PRELIMINARY ANALYSIS ON SIGNAL MONTE CARLO ψ(2s) WITH η π + π π 0 Figure 4.5: π 0 mass distribution 4.4 Pseudo η π + π reconstruction Concept The preliminary study showed that the neutral pion was at the origin of misreconstructions. One possibility is to ignore that particle and to consider pseudo η decay in π + π. ψ(2s) peak will undoubtedly loose resolution. However, it should be confined between M J/ψ +2M π and M ψ(2s) M π 0, between 3376 and 3551MeV Construction of the dataset and selection This dataset is constructed in a similar way than the first one. Only two changes are made: the mass windows for ψ(2s) and η are enlarged in order to accept the difference caused by the absence of π 0. This is summarized in MC Results for ψ(2s) with η decaying in 2 pions After generation of the set, a selection on the best candidate for each event is made. This is done by evaluating the χ 2 of the reconstructed vertex. The entry with the lowest value is kept. The resulting mass distribution for ψ(2s) is shown in figure 4.7. One can clearly identify a peak around 3500 MeV, a second one around 4500 MeV and combinatorial background in between them. The first peak corresponds to the signal: it is indeed contained between 3375 and 3550MeV. As expected, no event is observed below 3375MeV. Pions-kaons The peak at 4500 MeV is more surprising. Other particles must have been taken for pions. As the data comes from MC signal, the miss identified particles most likely come from B 0 s decays. Next, we consider the possibility for kaons in φ K + K are miss-identified as pions. One way to verify this is to study the PID variable. At Brunnel level, based on information from the different detectors - RICH among others, analysis provides the particle ID with a certain level of confidence on the assertion. The level of confidence that the particle is indeed a kaon, relative to a pion, is characterized by the Delta Log Likelihood variable, shortened DLLka.

13 4.4. PSEUDO η π + π RECONSTRUCTION 13 Figure 4.6: Cuts applied for η π + π 1000 htemp Entries Mean 3980 RMS Figure 4.7: ψ(2s) mass distribution for η π + π after the best candidate selection This is illustrated in the following figures: in 4.8 and 4.9, one can clearly observe an excess of pions with DLLka > 0 that explains the peak around 4500 MeV. Figure 4.10 shows the same distribution but for DLLka < 0, corresponding to actual pions. The ψ(2s) mass distribution for η π + π has a clear peak with little background. Proceeding with partially reconstructed η looks promising.

14 14CHAPTER 4. PRELIMINARY ANALYSIS ON SIGNAL MONTE CARLO ψ(2s) WITH η π + π π 0 :pi_21_dllka {pi_21_dllka>-150 && pi_22_dllka>-150} pi_21_dllka Figure 4.8: ψ(2s) mass distribution versus the Delta Likely Hood of π + {pi_21_dllka>0 && pi_22_dllka>0} 350 htemp Entries 9083 Mean 4215 RMS Figure 4.9: ψ(2s) mass distribution for pions with DLLka > 0 {pi_21_dllka<0 && pi_22_dllka<0} 500 htemp Entries Mean 3874 RMS Figure 4.10: ψ(2s) mass distribution for pions with DLLka < First Conclusions The η π + π attempt led to a conclusive result: graph 4.10 shows a clear peak within the expected mass window. This mode of proceeding will be adapted for MC signal and real data in the following sections. For all further analysis containing pions, their DLLka value

15 4.5. FIRST CONCLUSIONS 15 will be required to be negative. η π + π π 0 and η γγ did not give any result at that stage of the study. Working with separate set of data for each decay and with many more events could provide results. This will be explored in the following chapter.

16 Chapter 5 Monte Carlo simulation studies This section presents the core of the study. The previous section introduced leads for exploration of the disintegration of ψ(2s). Here, each channel is considered separately. Two data sets are generated, one for when η is decaying in pions, the other for η decaying in photons. 5.1 η π + π As preliminary study showed hopeful result for this pseudo decay, it is the first considered here Data Dataset is generated as explained in section 4.1. Monte Carlo data were generated for the decay Bs 0 φ ψ(2s), where ψ(2s) decays into J/ψ η. Again, MC2010 at 7 TeV in center of mass with the full detector is used for comparison with real data. The differences with the preliminary analysis dataset is that only the 3 pions decay channel is considered and events are used. Input particle lists are the same as for the preliminary study. Again, a loose preselection is made on the entries, mainly on the transverse momentum of the particles and the quality of the vertices. This is reported in table 5.1. A selection on the best candidate is made, keeping only the candidates with the best quality for the vertices. This constitutes the dataset used for the analysis Selection cuts In order to optimize the detection of ψ(2s), the different parameters are studied one after the other. Graphics showing ψ(2s) mass in relation with the different parameters explain the choice made for the cut values. Cuts on J/Ψ MassWindow: preliminary studies showed that using mass constrained J/ψ, the mass distribution is very good and nothing further is to be done. Transverse momentum: figure 5.2 shows the mass distribution of ψ(2s) versus J/ψ transverse momentum. The background at higher masses for ψ(2s) comes from J/ψ particles with low transverse momentum. The cut value is hence fixed at 4 GeV and only entries with higher transverse momentum are kept. Cuts on η Momentum: looking at ψ(2s) mass distribution versus η momentum, one can observe in figure 5.3 that low momentum events seem to be equally distributed. In order to 16

17 5.1. η π + π 17 Figure 5.1: Preselection criteria for the construction of the dataset for η π + π π 0 and partially reconstructed η :jpsi_1_pt jpsi_1_pt Figure 5.2: ψ(2s) mass distribution versus pt(j/ψ) for η π + π reject that evenly distributed background, the lower limit for η momentum is fixed at 10 GeV. Cuts on pion particle identification variable Particles identity: preliminary study showed that kaons tended to be taken for pions. As tight pions were taken as input particle lists, one can expect less confusion on the

18 18 CHAPTER 5. MONTE CARLO SIMULATION STUDIES :eta_2_p eta_2_p Figure 5.3: ψ(2s) mass distribution versus p(η) for η π + π identity. Indeed, the particle list restriction includes a selection on the likelyhood of the particle identity. Figures 5.4 and 5.5 show the relation between ψ(2s) mass and pions DLLka. DLLka can still reach positive values. In order to ensure the particles identity, the superior cut value on the DLLka is fixed at -1.

19 5.1. η π + π 19 :pi_21_dllka pi_21_dllka Figure 5.4: ψ(2s) mass distribution versus DLLka(π + ) for η π + π :pi_22_dllka pi_22_dllka Figure 5.5: ψ(2s) mass distribution versus DLLka(π ) for η π + π Result and interpretation The selection criteria applied on the dataset are summarized here: p t (J/ψ) 4 GeV p(η) 10 GeV DLLka(π) < 1 Selection criteria from MC study for η π + π The resulting mass distribution for ψ(2s) is showed in figure 5.6. One can clearly observe similarities with figure 4.10: the peak is within the expected mass window, no signal is observed under 3400 MeV and above the upper limit, only flat background remains. Fitting function As the mass distribution is not a simple peak but spread within a mass window, it is not only the effect of resolution that causes the distribution. The mass distribution is then fitted to the best by using two gaussians and a phase-spaced motivated distribution for the

20 20 CHAPTER 5. MONTE CARLO SIMULATION STUDIES Psi_Mass Psi_Mass Entries 2683 Mean 3545 RMS χ 2 / ndf / 41 Prob p ± p1 135 ± 53.2 p ± 23.5 p ± p ± 2.6 p ± 1.88 p ± p ± 1.1 p ± Figure 5.6: ψ(2s) mass distribution for η π + π background. As the peak is asymmetric, the first gaussian describes the slow progression on the left and the second one, sharper, the steep descent around the upper limit (around 3550 MeV). The background is described by a distribution with an end point at the lowest possible ψ(2s) mass M J/ψ + 2M π. The fitting function used is: a 1 f(x) = exp { (x µ 1) 2 } σ 1 2π + a 2 σ 2 2π exp + a 3 x c b (x 2σ 2 1 { (x µ 2) 2 } c 2σ 2 2 ) 2 1 exp { χ ( (x ) )} 2 1 c

21 5.2. η π + π π η π + π π Data For this decay channel, the exact same set of data is used than for the study of η π + π. It is described in subsection Input particles lists and preselection criteria are shown in table Selection cuts As for η π + π, the parameters are explored individually to optimize the selection. This section presents the justification behind each selection criterion. Cuts on J/Ψ Transverse momentum: figure 5.7 shows the relation between ψ(2s) mass distribution and p t (J/ψ). Rejecting the J/ψ particles with low transverse momentum diminishes the background for ψ(2s) high masses. The lower limit is fixed at 4 GeV. :jpsi_1_pt jpsi_1_pt Figure 5.7: ψ(2s) mass distribution versus p t(j/ψ) for η π + π π 0 Cuts on η MassWindow: figure 5.8 shows the mass distribution of η. Preliminary study showed that the presence of neutral pions prevent a good definition of η. A constrain of 30 MeV on M η is nevertheless applied. This is loose enough to tolerate incertitude on neutral pions and tight enough to reject misreconstruction of π 0. Momentum: by selecting entries with p(η) higher than 20 GeV, most of the background at heavy ψ(2s) is rejected. This can be seen in figure 5.9. This selection is radical: only a fourth of the sample passes it. Relaxing this cut has been tried without success. It is critical to reject an important part of the background and show the signal. Transverse momentum: in a similar way to p(η), a selection is made on the transverse momentum. Entries with a value higher than 1 GeV for p t (η) are kept. The corresponding graphic is shown in figure 5.10.

22 22 CHAPTER 5. MONTE CARLO SIMULATION STUDIES eta_2_mass 35 htemp Entries 1607 Mean RMS eta_2_mass Figure 5.8: M η distribution for η π + π π 0 :eta_2_p eta_2_p Figure 5.9: ψ(2s) mass distribution versus p(η) for η π + π π 0 3 Cuts on pions Particles identity: as in preliminary study or for the decay η π + π, a selection is made on the particles identity likelihood of the pions. Pions with a DLLka value inferior to -1 are kept. Figure 5.11 illustrates this for π +.

23 5.2. η π + π π 0 23 :eta_2_pt eta_2_pt Figure 5.10: ψ(2s) mass distribution versus p t(η) for η π + π π 0 :pi_21_dllka pi_21_dllka Figure 5.11: ψ(2s) mass distribution versus DLLka(π + ) for η π + π π Result and interpretation The criteria applied for the selection of ψ(2s) via the mode η π + π π 0 are summed up here: p t (J/ψ) 3 GeV M η MeV p(η) 20 GeV p t (η) 1 GeV DLLka(π) < 1 Selection criteria from MC study for η π + π π 0 This leads to the ψ(2s) mass distribution in figure It slightly shows a peak between 3685 and 3700 MeV. The number of remaining events is very limited: only 171 entries for an initial set of events. Working with a larger set of data would allow the conduct of more conclusive studies.

24 24 CHAPTER 5. MONTE CARLO SIMULATION STUDIES 16 htemp Entries 171 Mean 3710 RMS Figure 5.12: ψ(2s) mass distribution after selection for η π + π π η γ γ Data This dataset has been generated separately to the one used for the precedent section. In this case, η decays into photon pairs. The set comprises events. The method though remains identical: Monte Carlo data were generated for the decay B 0 s φ ψ(2s), where ψ(2s) decays into J/ψ η. Again, MC2010 at 7 TeV center of mass is used for comparison with real data. Input particle lists are mass constraind J/ψ to µ + µ and standard loose photons. A loose preselection is made on the entries, this is reported in table A selection on the best candidate is made, keeping only the candidates with the best quality for the vertices. This constitutes the dataset used for analysis Selection cuts Cuts on J/Ψ Transverse momentum: figure 5.14 shows ψ(2s) mass distribution versus p t (J/ψ). The cut value is fixed at 4 GeV, suppressing an important part of the background at high masses. Cuts on η Momentum: The cut value is fixed at 15 GeV. It is once more the selection on η momentum that removes an important part of the background. Indeed, the excess of events at low momentum is responsible for the important quantity of entries with a too heavy mass for ψ(2s). This can be observed in figure Transverse momentum: for identical reasons, only entries with p t (η) higher than 1 GeV are kept. It could seem excessive from observation of the figure 5.16, but it is necessary to ensure a good separation between signal and background. Indeed, this criteria suppresses an important part of the background above 3700 MeV for M ψ(2s) distribution.

25 5.3. η γ γ 25 Figure 5.13: Preselection criteria for the construction of the MC generated dataset for η γγ :jpsi_1_pt jpsi_1_pt Figure 5.14: ψ(2s) mass distribution versus p t(j/ψ) for η γγ Result and interpretation The criteria applied for the selection of ψ(2s) via the mode η γγ are summed up here:

26 26 CHAPTER 5. MONTE CARLO SIMULATION STUDIES :eta_2_p eta_2_p Figure 5.15: ψ(2s) mass distribution versus p(η) for η γγ :eta_2_pt eta_2_pt Figure 5.16: ψ(2s) mass distribution versus p t(η) for η γγ p t (J/ψ) 4 GeV p(η) 15 GeV p t (η) 1 GeV CL(γ) 0.5 Selection criteria from MC study for η γγ The resulting mass distribution is shown in figure An important part of the background has been suppressed, but no signal can be really observed. As feared, the γγ channel is subject to too many combinations, drowning the signal under misreconstruction. Nothing much can be hoped for real data. A new attempt could though be tried with a larger set of data.

27 5.3. η γ γ htemp Entries 341 Mean 3746 RMS Figure 5.17: ψ(2s) mass distribution after selection for η γγ

28 Chapter 6 Real Data In this chapter, previous criteria for selection of ψ(2s) are applied on real data for the three decay modes. The results are compared with Monte Carlo generated data. Real data origin is detailed and a short calculation on branching fraction is made. 6.1 Data The data used is Stripping10. It corresponds to a set of treated data acquired in 2010 at 7 TeV center of mass. 6.2 η π + π Selection cuts Analysis conducted on the MonteCarlo dataset led to the following criteria for the selection of η π + π : p t (J/ψ) 4 GeV p(η) 10 GeV DLLka(π) < 1 Selection criteria from MC study for η π + π As it has already been optimized, it is applied on real data without a change Result The resulting mass distribution of ψ(2s) is shown in figure 6.1. No signal between 3400 and 3550 MeV can be identified. The curve is dominated by background. However, around ψ(2s) mass at 3686 MeV, a small but sharp peak is observed. This corresponds to the decay ψ(2s) J/ψ π + π. Indeed, in real data, not only the designated decay channel is present. As the direct decay mode is ten times more important (table 3.1), it is not surprising that it shows through first. 28

29 6.2. η π + π 29 Psi_Mass Psi_Mass Entries Mean 3654 RMS 92.2 χ 2 / ndf / 91 Prob 9.966e-08 p0 242 ± 11.5 p ± 0.7 p ± 0.3 p e+04 ± 3.121e+03 p ± 1.4 p ± 1.00 p ± 99.3 p ± 0.9 p ± Figure 6.1: ψ(2s) mass distribution with η π + π for real data, where the selection criteria are determined by MC study Investigation on ψ(2s) J/ψ π + π In order to confirm that the peak is indeed ψ(2s) J/ψ π + π signal, the cuts are reinforced: p(j/ψ) 100 GeV p t (J/ψ) 6 GeV p(η) 15 GeV p t (η) 1 GeV DLLka(π) < 1 Selection criteria for η π + π reinforced for real data The resulting mass distribution is shown in figure 6.2. One can clearly see the confirmation that the peak at 3686 MeV is ψ(2s) J/ψ π + π signal. One can assume that both channels, ψ(2s) J/ψ π + π and ψ(2s) J/ψ η, are affected by the selection in a similar way, meaning that they have the same selection efficiency.

30 30 CHAPTER 6. REAL DATA Psi_Mass Figure 6.2: ψ(2s) mass distribution with η π + π for real data. The criteria are reinforced to underline the peak at 3686 MeV. With this hypothesis and evaluating the number of ψ(2s) J/ψ π + π by fitting the distribution, the number of ψ(2s) J/ψ η could be approximated. This calculation is presented here, after a short explanation on the fitting functions.

31 6.2. η π + π 31 Fitting function The fitting functions are of the same form as the one used for MC data in section Only this time, the gaussian and the phase-space motivated function describe the background. The ψ(2s) J/ψπ + π signal is described by a narrow gaussian. a 1 f(x) = exp { (x µ 1) 2 } σ 1 2π + a 2 σ 2 2π exp + a 3 x c b (x 2σ 2 1 { (x µ 2) 2 } c 2σ 2 2 ) 2 1 exp { χ ( (x ) )} 2 1 c Approximation on the number of ψ(2s) J/ψ η events From the fitting functions, the number of events for the ψ(2s) J/ψπ + π channel is of ± 99.3 for the figure 6.1. This number is to take with precaution as the gaussian is very narrow. As the channel for ψ(2s) J/ψ η is ten times less frequent, there might be approximatively 40 ψ(2s) J/ψ η events. As in the present case partially reconstructed η are considered, the signal is spread over 175 MeV, there is no chance to observe a statistical distribution of the decay.

32 32 CHAPTER 6. REAL DATA 6.3 η π + π π 0 As for the precedent mode, the criteria determined by the study of the MC data is applied to the real data for η π + π π Selection cuts The cuts determined by MC study are summed up here: p t (J/ψ) 3 GeV M η MeV p(η) 20 GeV p t (η) 1 GeV DLLka(π) < 1 Selection criteria from MC study for η π + π π Result and interpretation htemp Entries 1335 Mean 3741 RMS Figure 6.3: ψ(2s) mass distribution with η π + π π 0 for real data, where the selection criteria are determined by MC study The resulting mass distribution for ψ(2s) is shown in figure 6.3. No significant signal is observed.

33 6.4. η γ γ η γ γ Selection cuts The criteria for the selection of this mode have been determined with a mild success in section 5.3. They are summarized in the following table: p t (J/ψ) 4 GeV p(η) 15 GeV p t (η) 1 GeV CL(γ) Result and interpretation Selection criteria from MC study for η γγ Applied to real data, the selection leads to the result presented in figure 6.4. As for η decaying in three pions, the mass distribution of ψ(2s) is dominated by background. What differs is the number of remaining entries after selection. For the 3 pions mode, only a thousand of entries were left. In the present decay, this number is ten times more important. But the quantity of entries close by ψ(2s) mass is still limited. A large quantity of background is still present at high masses, betraying that the selection is probably not optimized. 450 htemp Entries Mean 3782 RMS Figure 6.4: ψ(2s) mass distribution with η γ γ for real data, where the selection criteria are determined by MC study

34 Chapter 7 Conclusion The goal of this work was to study the decay of ψ(2s) via the mode J/ψ η, where η could decay in two photons or π + π π 0. The first part of the work was realised on MonteCarlo generated data, the second, on real data. The latest did not lead to any conclusive result. This could have been foreseen by studying the branching fractions of the considered decays. ψ(2s) J/ψ η is indeed to infrequent in comparison with the quantity of available data. Concerning Monte Carlo generated data, the study of ψ(2s) via the γγ channel did not lead to any conclusive result. This could be due to combinatorial background. Nevertheless this channel does not seem promising. On the other hand, η decaying in pions seems like a more promising channel. η π + π π 0 showed a signal peak in MC data for tight criteria. More interestingly, ignoring the π 0 and considering partially-reconstructed η π + π turned out to be fruitful. Indeed, the selection was more effective to separate signal from background. This is definitively something to try on more important set of real data. 34

35 Chapter 8 Annexe Figure 8.1: Monte Carlo Truth information on the particles transverse momentum. From top to bottom, p t(ψ(2s)), p t(j/ψ), p t(π 0 ) 35

36 36 CHAPTER 8. ANNEXE Figure 8.2: Monte Carlo Truth information on the particles momentum. From top to bottom and left to right, p(η), p(j/ψ), p(ψ(2s)), p(π ), p(π + ), p(π 0 )